Electric Probe Diagnostics of Thermal Plasma Jet at Low Pressures

Introduction The possibility of application of electric probes for diagnostics of thermal plasma jets is limited, especially due to the interaction of the probe with plasma. Nevertheless, some important information can be obtained. Electric probes were used for measurement of plasma potential in arc plasma [Chapman and Jones, 1978; Hrabovský and Konrad, 1991; Popov and Gateval, 2001], and for determination of structure of plasma jet boundary [Hrabovský and Kopecký, 2005]. In this paper we will use the probes for diagnostics of expanding thermal plasma jet at low pressures at larger distances from the plasma torch exit, where plasma radiation is very week and application of optical methods of diagnostics commonly used in thermal plasma investigations is not possible. Although this region of plasma jet is very important for plasma processing, especially for vacuum and low pressure plasma spraying, the possibilities of its diagnostics are limited. The goal of presented measurements is determination of possibilities of application of electric probes for diagnostics of expanding and decaying plasma jets.